It has been known for some time that molecules of various molecular weights can be separated across a semi-permeable membrane. The membrane by virtue of its composition, and consequently its porosity, allows molecules equal to or less than a particular molecular weight to cross the membrane. Larger molecules are unable to cross. This has led to four common applications of dialysis membrane: 1) exchanging one sample buffer for another buffer, 2) sample desailing, 3) molecular separations, and 4) sample concentration. These applications are most often utilized in the area of laboratory research and the dialysis of patient bodily fluids such as blood.
Various methods have been developed so that dialysis membrane is the sole pathway of molecular exchange between a sample and dialysate. The most widely used method in the research laboratory is taking the dialysis membrane which is molded in the shape of a tube and tying, or clamping, one end of the tube to form a sack. The sample solution is added to the interior of the dialysis membrane sack which is then tied or clamped at the other end which had remained open. The sack, now a closed vessel, is submerged into the dialysate.
The method described above has significant drawbacks. The tying or clamping of the ends of the dialysis membrane tubing requires skill. If the end of the tubing is not carefully tied, the sack will leak and the sample can be lost. Also, it is difficult to load and unload the sample from the sack because the membrane is flaccid; samples are often spilled during these steps. Touching the dialysis tubing membrane with fingers can also effect the sample dialysis. Therefore it requires skill to touch as little of the membrane as possible when tying or clamping it. An alternative is to wear gloves, however, it also requires skill to tie the tubing while wearing gloves. Since the sample chamber of the dialysis tubing membrane is open during the loading and unloading of sample, the sample can be contaminated with any substance in the environmental air. It would be desirable to have a sample chamber which is hermetically sealed and to add the sample with a device such as a needle and syringe. Also, wetted dialysis membrane tubing can not be labeled so labeling must be written on a small clamp or on an object which is inconveniently attached to the tubing with material such as string.
In order to address some of the problems with loading sample into and unloading sample from dialysis tubing as described above, one company has offered commercially preformed dialysis sacks. These sacks are dialysis tubing which has already been clamped at one end and at the open end a funnel has been attached. After the sample is loaded through the funnel, the tubing is clamped below the funnel and dialysis proceeds. Although the loading and unloading of sample are somewhat simplified, the product still suffers the other problems as described above for dialysis tubing.
Another commercially available product has taken another approach to addressing some of the inconvenience of the dialysis tubing and the pre-formed dialysis tubing sack. Two concentric rings, one larger than the other, trap a sheet of membrane between the rings when the outer ring is tightened upon the inner ting. A vessel is formed such that the rings form the walls of the vessel and the floor is the dialysis membrane. The vessel then is floated on top of the dialysate and sample is added to the interior of the floating vessel. Although this solution offers advantages, it introduces new problems. First, the sample is open to the environmental air which allows it to be easily contaminated. Secondly, because the vessel is open, it is easy for the sample to spill into the dialysate as it floats. Loading and unloading are greatly simplified, but assembly of the device requires some skill by the user.
Various patents have issued for the dialysis of patient bodily fluids. For example, U.S. Pat. No. 3,459,176 describes a device for the dialysis of human blood. The invention embodies an inner chamber which dialyzes with dialysate which is contained within a rigid and fixed outer chamber. The inner chamber, or sample chamber, is open on both ends which allows it to be extended by tubing and joined to the patient's circulatory system for dialysis with the dialysate. This device does not allow the sample chamber to be moved from the fixed dialysate vessel to another dialysate vessel. Also, since the sample chamber is designed to be made continuous with the patient's circulatory system the chamber does not accommodate a small, fixed-volume sample such as that used in the research laboratory.
A similar flow system to that described above is shown by Wyatt et al. in U.S. Pat. No. 3,679,059 as a gasket separator unit for membrane packs and a process for the preparation of same. The device embodies a mesh lattice which is embedded a the edges with gasket-like material to form a unit. These units are then stacked in multiples to form a pack. Dialysis, or similar, membranes are fixed between the mesh lattices during the formation of the pack and are accessed through ports in an arrangement described in U.S. Pat. No. 2,758,083. The result is a series of chambers that are fixed in relation to each other and contain alternately sample and dialysate as initially described by Heibig in U.S. Pat. No. 1,757,364. Both the sample and dialysate flow through the series of chambers in opposite directions in order to realize dialysis of both solutions in their enshrines. The sample chamber can not move freely within a dialysate vessel of the user's choice or be easily transferred to another dialysate vessel. This makes the membrane pack impractical for the dialysis of small, fixed-volume research samples.
Urbain described in U.S. Pat. No. 1,777,057 a system for the dialysis of putrescible liquids which practices a fixed-volume sample chamber and a sealed dialysate vessel. This is achieved by placing the putrescible liquid in a sample chamber and floating the sample chamber within a sealed outer vessel which contains the dialysate. The sealed dialysate vessel is ganged in conjunction with other vessels which allow only the introduction of oxygen-free nitrogen gas. Although Urbain practices the use of a sealed dialysate vessel, for use in the research laboratory it would be desirable to have a sealed sample chamber. This would allow the sample being dialyzed to easily be moved freely from one dialysate vessel to another, or to easily change the dialysate within the outer vessel.
Another device has been described by Leon (U.S. Pat. No. 4,865,813) which embodies a sample chamber for a fixed volume sample. Leon shows a sealed sample chamber with a septum for the introduction of sample into a sample chamber with a needle and syringe. Surrounding the sample chamber are four fixed reagent chambers which are communicable with the sample chamber in the center. Molecules from the sample chamber can diffuse into the four chambers, containing four different reagents, resulting in separate colorimetric reactions based on the presence of unknown analyte being analyzed. Since the reagent chambers are presealed for storage, the device is not practical for dialysis of samples such as those dialyzed in the research laboratory. The dialysate chambers are not only in a fixed geometry to the sample chamber, but they are of fixed volume which is not accessible for the addition of fresh dialysate.
U.S. Pat. No. 3,696,931 describes a device which is used for the purification of sea water or water containing impurities. The invention embodies a closed chamber. The closed chamber contains matter which is used to draw water osmotically through a semipermeable membrane leaving contaminants out. The chamber is not hermetically sealed and sample cannot be loaded and unloaded with a device such as a needle and syringe.
Of the devices described above, none permit the convenient loading and unloading of small, fixed-volume samples to be dialyzed in the research laboratory. Also, none protect the sample from contamination during loading and unloading.